From the standpoint of downsizing a vehicle and enhancing the safety, the airbags are demanded to deploy at a higher speed. Among others, in a side curtain airbag, the space between the vehicle body and the crew is small, as compared with the driver's airbag or the like and therefore, a higher deployment speed is required. For satisfying this requirement, it is necessary to reduce the weight of the bag body and minimize the leakage of the inflating-gas. Also, in a collision, even when the bag body is collapsed by the contact with the crew and the internal pressure thereof becomes higher, the airtightness of the bag body must be maintained. If the airtightness is not maintained, the deployed bag cannot receive the occupant, and the crew may be put into contact with the vehicle body and injured. That is, there is demanded a bag in which the pressure achieved at deployment (ultimate deployment pressure) is high. Furthermore, in order to quickly catch the occupant in a narrow space, i.e., to enable restraint in a limited short distance, a quick restraint property of quickly catching and restraining the occupant is demanded.
In order to realize compactness and lightweighting of an airbag, Patent Document 1 discloses use of a yarn having a low fineness and a high tenacity. However, in practice, when the crew is contacted with the bag, a stress is concentrated on the boundary portion between the inflating part and the non-inflating part and if the tenacity of the original yarn is merely increased, fibrillation of the fiber proceeds and the strength in the direction perpendicular to the fiber axis is decreased, as a result, breakage occurs in the boundary portion between the inflating part and the non-inflating part, failing in maintaining a practically sufficient bag strength. Above all, in an application requiring higher-speed deployment as in a side curtain air bag, there is a problem that the bag may burst.
As the method for improving airtightness of the bag body, Patent Document 2 discloses a method where the single filament fineness of a weaving thread is reduced to be smaller than conventional threads and the average dynamic air permeability and dynamic air permeability curve index of a plain-wovenbase fabric are set to 500 mm/s or less and 1.5 or less, respectively. However, the portion actually governing the deployment speed of the bag body is the boundary portion having a large air permeation amount, i.e., the sewn part, between the inflating part and the non-inflating part, and even when the air permeation amount of only the base fabric is reduced, which is not sufficient.
Also, Patent Document 3 discloses a hollow-woven airbag base fabric in which the air permeability in the boundary portion between the multilayered cloth part (inflating part) and the part which does not inflate (non-inflating part) of the hollow-woven fabric is set to 0.25 liter/cm/min or less under a differential pressure of 50 kPa. However, the portion actually governing the deployment speed of the bag body is the joining part of the hollow-woven fabric, where the occupant corner into contact with the deployed bag and a higher stress is thereby imposed, and under a more practical load condition, opening may be generated in the boundary portion between the inflating part and the non-inflating part at deployment depending on the yarn characteristics, leading to a large air permeation amount and a low deployment speed.
Furthermore, Patent Document 4 has proposed to control the elongation under a load of 50 N/cm of the base fabric to 15% or less and the elongation under a load of 300 N/cm of the base fabric to 30% or less in order to increase the airbag deployment speed and at the same time, control the elongation under a load of 300 N/cm of the base fabric to 15% or more so as to reduce the impact force generated when the occupant is out into contact with the air bag. However, even with these physical properties of the base fabric, a sufficient effect may not be brought out in a side curtain air bag which is required to deploy at a higher speed. In other words, the characteristics of the boundary portion between the inflating part and the non-inflating part, which is subjected to stress concentration when loaded, are important at actual deployment. Detailed studies on this portion have not yet been made and no practical technique has been proposed so far. In particular, a technique satisfying all of compactness, burst resistance and high-speed deployment has not yet been proposed.